The value of the Dex Code model is the sum of the original uncompressed file sizes of all loaded dex files. It is calculated using the best approximation of the dex file size available to us on device. On Android O, for example, this can be approximated as the virtual size of the corresponding memory mapped .vdex file read from /proc/self/maps. Different Android platform versions and scenarios may require different approximations.
The actionable breakdown of the dex code model is a breakdown by dalvik.system.DexFile instance. Further breakdown of individual dex files can be achieved using tools such as dexdump.
For example, for an application AmmTest.apk that has a single classes.dex file that is 500 KB uncompressed, the DexFile instance for /data/app/com.android.amm.test-_uHI4CJWpeoztbjN6Tr-Nw==/base.apk is shown as Taking up 500 KB (or the best available approximation thereof).